1. Field of the Invention
This invention relates to the elimination of air pollution due to sulfur dioxide in industrial sour gases emitted into the atmosphere. The process is a continuous cyclic one, employing aqueous solutions in both the sour gas scrubbing and in the chemicals recovery steps. Sulfur dioxide is scrubbed out of sour gases by means of a reactive aqueous solution; sulfur is then recovered from the solution as hydrogen sulfide in a process which also receives the reactive solutions.
2. Brief Description of the Prior Art
Numerous processes have been proposed or are in various stages of development or use to abate sulfur dioxide pollution of the atmosphere. These may be classified generally as employing a wet or a dry process.
Wet processes have received most of the attention, partly because at least a part of the technology needed was already available. These include throw-away processes which scrub the gases with a lime, limestone, or dolomite slurry and discard the spent slurry. A variation of this throw-away process is to scrub the gases with an alkali metal absorbent solution, and to regenerate the alkali by reaction with lime. The spent lime is discarded.
Other wet process employ sodium or ammonium-based absorbent solutions. The ammonium-based solutions are not readily recoverable. One sodium-based plant, in limited commercial operation, delivers sulfur dioxide which can be reduced to elemental sulfur if desired. Others are in various stages of development and will produce sulfuric acid or elemental sulfur.
Some novel processes still in early stages of development include a formate process using a potassium salt, a citrate process, and a process using an undisclosed organic solvent, to cite a few examples.
Dry processes are generally in early development stages and include the use of activated carbon, or molten salts as the active sulfur dioxide removal agent.
At one time the use of potassium sulfite operated in a sulfite/bisulfite cyclic process was advocated because, with the potassium salt, which has a higher solubility than the sodium salt, less water would have had to be evaporated in the recovery step and thus recovery cost would have been lower. In the development of the process, however, two unforeseen but important difficulties were encountered. In the absorber, potassium metabisulfite was formed in addition to potassium bisulfite when sulfur dioxide in the gas reacted with potassium sulfite in the solution. The metabisulfite, which is the least soluble of the three compounds, crystallized out of solution and plugged the tower. In addition to this problem, a much larger amount of potassium sulfate than predicted was produced in the recovery step. A market for this product was dubious and the cost of replacing potassium lost as a sulfate was too high for the process, hence the use of potassium sulfite as an absorption medium in this type of process was abandoned.
All of the processes in use or being developed have their advantages and disadvantages. None of them stands out as the probable candidate for wide spread, long-term applications, but the need for such process is clear.
3. Objectives of this Invention
One objective of this invention is to provide a cyclic wet scrubbing process for the removal of oxides of sulfur from gases. Another object is to provide a cyclic process employing a reactive aqueous absorption solution which effectively removes oxides of sulfur from gases, regenerates the aqueous absorption solution for reuse in the gas cleaning stage, and produces a sulfur compound readily convertible into elemental sulfur.
These and other objectives will become apparent in the following description of the invention taken with reference to the drawings.